1. Field of the Invention;
The invention pertains generally to a circuit arrangement for the compensation of reactive power in a power system and more particularly to a circuit arrangement for the compensation of reactive power having protective devices in case of misfiring by the circuit.
2. Description of the Prior Art:
Circuit arrangements of this general type are known in the prior art as can be seen in an article by H. Frank and S. Ivener, "Statische Blindstromkompensation in der elektrischen Stromversorgung" [Statical Reactive Power Compensation in Electrical Power Supply] appearing in the house organ ASEA-ZEITSCHRIFT 1981, Vol. 26, No. 5-6, pp. 113-119.
This prior art device includes a thyristor rectifier contained in the circuit arrangement which is simply constructed from two anti-parallel thyristors or from two antiparallel-connected thyristors in a series circuit, to whose control electrodes a control circuit is connected with ignites or triggers the thyristors. This occurs when there is a need for improvement of the reactive power balance or for reasons of voltage maintenance in the alternating-current voltage system. Triggering should only be accomplished at moments when the instantaneous value of the alternating-current voltage specifically displays a minimum at the thyristor rectifier in order to prevent damage to the thyristors.
Triggering of the thyristors at any other moment can have serious consequences for the thyristors, up to and including their destruction. Since such misfires cannot be wholly avoided, an inductor and a discharge or grounding device provided for the protection of the thyristors in the prior art circuit arrangement, with this discharge device being connected directly above the thyristor rectifier. The inductor has the task of limiting the transfer current flowing across the capacitor following a misfire. The discharge device has the task of preventing an overvoltage condition as soon as the previously conductive thyristor has again switched back to the off-state. This overvoltage would, without the effect of the discharge device reach a value equal to the difference beween the capacitor voltage and the system voltage, aned could amount to more than triple the amplitude of the system voltage. The overvoltage would be particularly critical for the previously misfired thyristor, since it may be greatly overheated from the previous transfer current and therefore be substantially reduced in its overvoltage resistance.
The disadvantage of this prior art circuit arrangement, however, is that the inductor and the discharge device afford protection only against a single misfire. In the event of a double misfire, when a second misfire occurs shortly after a first one, the destruction of the thyristor rectifier can still occur in the prior art circuit system.